Vehicular Ad-Hoc Networks rely on a multi-channel architecture to support vehicle-to-vehicle and vehicle-to-infrastructure communications. Multiple service channels are assigned in the 5GHz spectrum for non-safety data transfer, while a unique control channel is used for broadcasting safety messages and service advertisements. Single-radio vehicular devices stay tuned on one radio channel at a time and alternately switch between channels to monitor safety messages and to access information and entertainment services; while dual-radio devices can simultaneously stay tuned on both types of channels. Multi-channel coordination, synchronization, and access are big challenges in VANETs; many design choices are still open issues in ETSI and IEEE standardization bodies. In this paper, counter-measures and recent trends in standardization bodies are discussed to cope with inefficiencies related to multi-channel operation for single-radio devices (
inefficient spectrum utilization, synchronized frame collisions, bandwidth waste), and dual-radio devices (
cross-channel interference, coexistence with single-radio devices).